This invention is related to the field of the CIS (Contact Image Sensor) technology, in particular, it concerns the manufacturing of a sensor chip and the assembly of a CIS module with a butting technique to form a sensor chip array.
The prior art technique of butting for the assembly of a sensor array inside a CIS module is schematically illustrated in FIG. 1 wherein a PCB (Printed Circuits Board) 100 is shown. Sensor chips 110, 120, . . . , 190 were attached to the PCB 100 and butted to form a linear array. All the sensor chips 110, 120, . . . , 190 were of the same design and were manufactured with the same process. That is, the pixel arrays (1101 to 1109), (1201 to 1209) and (1901 to 1909) were the same. Likewise, the mux (multiplexing) switch arrays (1111 to 1119) from chip 110 and (1911 to 1919) from chip 190 were the same. In each of the sensor chips 110, 120 and 190, each switch of the mux switch array was connected between a corresponding pixel and a single common line. For example, in chip 110, the common line is designated as 1121. In turn, the common line 1121 was connected to an output bonding pad 1132. Other bonding pads,: like bonding pad 1131 in chip 110, are shown with no connections on purpose, as they are not relevant to the current invention.
The output bonding pad 1132 from chip 110 and the other output bonding pads from the other chips were wire bonded to a common conductor stripe 1151, which in turn was connected to an associated electronic block 1173 necessary for the proper functioning of the CIS module. The detail of the electronic block 1173 is not shown here as it is not relevant to the current invention. Additionally, if the pixel of the sensor array was of a photo-transistor type, the common conductor stripe 1151 was connected to a charge integrating capacitor 1161 and an input resistor 1162, which in turn was connected to the non-inverting input terminal of an operational amplifier (OP) 1170. However, if the pixel of the sensor array was of a photo-diode type, the charge integrating capacitor 1161 can be omitted from the circuitry. The output terminal 1171 of operational amplifier 1170 was connected to the electronic block 1173 for final output of the photo-signal from each pixel. A feedback resistor 1172 was connected between the output terminal 1171 and the inverting input terminal of the operational amplifier 1170. A gain-control resistor 1175 was connected between the inverting input terminal of the operational amplifier 1170 and ground. The operational principle of the sensor array can be described as follows:
After a desired time period of exposure of the sensor array to an incident light, the generated light-signal from each pixel was read by applying a read signal pulse to turn on an individual switch of the mux switch array in sequential order from left to right of each chip. After the light-signal from the last pixel of the first chip 110 was read, the first pixel 1201 of the next butted chip 120 was read and so on until the reading of the light-signal from the last pixel 1909 of the last chip 190 to complete the reading of light-signal from the entire sensor array on the PCB 100.
Next, the process of generation of the light-signal from a pixel and its readout is described in more detail. With the mux switch 1111 turned on, the charge integrating capacitor 1161 started to sense the light-signal from the first pixel 1101 by accumulating the photo-charge flowing from the first pixel. The light-signal from the first pixel 1101 of the first chip 110 was then amplified by the operational amplifier 1170 with a gain which was determined by the ratio of the feedback resistor 1172 to the gain-control resistor 1175. The amplified light-signal from the first pixel 1101 appeared at the output terminal 1171 of the operational amplifier 1170 and was transferred to the outside system through the associated electronic block 1173. After reading the light-signal from the first pixel 1101 of the first chip 110, the stored photo-charge of the charge integrating capacitor 1161 was cleared by applying a reset signal pulse to turn on a reset switch 1181 which was a transistor connected across the charge integrating capacitor 1161. The charge integrating capacitor 1161 was then ready to read the light-signal from the next pixel. Thus, a second read pulse was applied to turn on the second mux switch connecting the second pixel of the first chip 110 and the common conductor stripe 1121. The aforementioned reading process of the light-signal from the first pixel 1101 was repeated to acquire the light-signal from the second pixel. This reading process was continued until every pixel of the first chip 110 was read. After the light-signal from the last pixel 1109 of the first chip 110 was read, the first pixel 1201 of the second chip 120 was read following the same procedure as described above. This reading process was continued on until the last pixel 1909 of the last chip 190 of the chip array to complete the reading of all the light-signals of the sensor array. Likewise, the dark-signal, which was the signal from the pixel with no light exposure, was read from each pixel of the sensor array with the same process as described above for the reading of the light-signal. Finally, the actual usable photo-signal from each pixel was computed as the corresponding light-signal minus the dark-signal for the subject pixel.
While this technique is simple, it suffers from a drawback of high assembly cost as many components, like a charge integrating capacitor 1161, three resistors 1162, 1172, 1175 and an operational amplifier 1170, are required to be assembled onto the PCB 100. The result is increased cost of the CIS module.
In order to reduce the cost of the CIS module, an approach was taken to integrate the operational amplifier into the sensor chip. This is illustrated in FIG. 2. From now on, the same component designation will be used in different figures whenever either the same component or a component with the same function is encountered. As shown, the sensor chip 200 now included additional components of a charge integrating capacitor 210, a reset switch 281, an operational amplifier 231 plus two resistors, a feedback-resistor 252 and a gain-control resistor 253 in contrast to the conventional sensor chips 110, 120, . . . , 190 from FIG. 1. An output bonding pad 1132 was provided for the output-terminal 251 of the operational amplifier 231. Each operational amplifier functioned only while a light signal was read from the pixels within the same chip. Each chip had its own charge integrating capacitor for reading purposes. The pixel array (1101 to 1109) and the mux switch array (1111 to 1119) of the sensor chip 200 remained the same as those shown in FIG. 1. An associated electronic block 259 was also shown for other electronic functions. Thus, just like the chip array 110, 120, . . . 190 from FIG. 1, many of these chips 200 with their respective on-chip operational amplifiers 231 were butted to form a sensor array of the desired length. The operational principle remained the same as described in FIG. 1 except that each chip now has its own operational amplifier instead of a common operational amplifier being shared by the entire chip array. While the associated assembly cost of the CIS module was now reduced with the corresponding reduction of component counts, other problems were brought about by this approach. Firstly, the offset voltage of the operational amplifier was different from chip to chip. Additionally, the gain of the operational amplifier also varied slightly from chip to chip. This resulted in an undesirable non-uniformity of the dark signal level. Secondly, the required size of the charge integrating capacitor 210 was usually large. Consequently it was difficult if not impossible to include the charge integrating capacitor 210 on the sensor chip 200 without increasing the chip size. Thirdly, the capacitance of the charge integrating capacitor 210 could vary from chip to chip due to variation of the manufacturing process. This resulted in an undesirable non-uniform output signal level at the output terminal of the operational amplifier 231.
Therefore, the current invention is conceived to resolve these difficulties and to improve the performance of the sensor chip with an integrated on-chip operational amplifier.
The first objective of this invention is to provide a design of a sensor chip having an integrated operational amplifier to reduce the number of components in the assembly of a CIS module.
The second objective of this invention is to provide a technique which, while disabling all other unwanted operational amplifiers in the butted chip-array on a PCB, employs only one operational amplifier from one sensor chip in the chip array for reading light-signals from all pixels of the sensor array.
The third objective of this invention is to provide a technique to achieve a variable gain of the operational amplifier.
The fourth objective of this invention is to provide a technique to distribute the required large capacitance of the charge integrating capacitor over the sensor chips in the entire chip array.
The fifth objective of this invention is to provide a technique to read a light-signal from every pixel in a selected number of sensor chips within the chip array with the same operational amplifier.
The sixth objective of this invention is to provide a technique to connect a selected number of components in a chip array to one designated location on the PCB.